This invention relates primarily to flow indicating devices and more particularly to improved flow meters for parenteral solution administration wherein greater uniformity of flow rates are achieved regardless of viscosity or density of the solutions.
Parenteral administration sets are generally used to infuse various types of solutions into a patient. The administration set provides a sterile passage for a physiological fluid in a supply container, e.g. sterile water, saline, or various concentrations of glucose solution, etc. Conventional intravenous sets employ a closed drip chamber with a drip nozzle whose main function is to enable the rate of flow to be calculated by observing the number of drops per unit time. This is not only time consuming but inaccurate. On the one hand drop size changes with flow rate and on the other, conversion of drop timing to drip rate is difficult.
A number of flow meters have been designed in an attempt to improve on the accuracy and speed in determining flow rates. They generally employ a small ball of proper specific gravity positioned within a tapered vertical tube. The ball, which has a specific gravity greater than that of the fluid, rises to a larger cross sectional area of the indicating tube as the fluid flows upwardly in the tube. The position of the ball relative to a calibrated scale indicates the flow rate of solution through the set. Some rotameter devices employ a light weight float, but the principle of operation is the same.
The advantages of these prior art flow meter devices over conventional drip sets noted above, is that flow rate can be more accurately and quickly set. Furthermore, a change in flow is readily apparent during a long term infusion and can be readily adjusted by the nurse.
Major disadvantages of these flow meter devices are the high cost and the inability to manufacture the flow indicating tapered tube and ball to the required tolerances to achieve sufficient flow accuracy. High volume production is not feasible for moulding plastic parts to sub-thousandths of an inch accuracy requirements. Selective assembly of components has also been tried but results in excessive assembly time and cost.
A major performance disadvantage of these rotameter type flow meters lies in the fact that they are sensitive to fluid viscosity, density, and temperature. Consequently, these devices are generally only accurate for one type of fluid at a known temperature. This is due to the fact that the larger the fluid viscosity the larger the drag force on the ball and the larger the fluid density, the greater the momentum force across the ball. Increases in these parameters cause the ball to travel higher up the tube at the same flow rate, thereby producing error.
Temperature changes primarily affect the viscosity of solutions with the drag force increasing as temperature decreases. For these reasons fluid calibration curves are usually supplied with industrial type devices. This is not practical nor desired in hospital application. For example, a flow meter of this type will not read accurately for 20% glucose solutions which are quite viscous if the flow meter device had been calibrated for less viscous 5% glucose solutions.
Furthermore, dimensional changes in the typically plastic parts after moulding or during storage prior to shipment affects the accuracy of the device. Warpage or locked in stresses gradually relax inducing undesireable strains. Also, the complexity of present moving element flow meter devices results in a substantial number of parts, typically in excess of 20, which unavoidably adds to assembly time and increased cost to the point that most hospitals cannot afford the extra cost for other than certain specialty applications such as pediatric use.
Another type of flow meter device has been disclosed by D. S. Stevens in U.S. Pat. No. 2,479,786. His device comprises a glass tube bent back upon itself similar in shape to a "J" and with a hole in the wall of the tube at the bent portion leading into the shorter leg. The tube is encased in an enlarged tubular body having an outlet. A liquid flowing into the tube rises into the shorter leg of the tube and flows slowly through the hole in the bend and subsequently through the outlet. The height which the liquid reaches in the shorter leg is set by regulating a stopcock located between a solution supply container and the flow meter. Indicia on the shorter leg mark the various flow rates for the device. Although the design of this type flow meter is relatively simple as compared to the rotameter type described above, in tests conducted with a model constructed as disclosed, flow rates also substantially varied depending on the viscosity of the solutions used. A major problem with this flow meter is the inability for making holes of uniform size and shape in the glass tube of large numbers of the flow meter, thus substantially affecting the calibration of one device to the other. It would be difficult if not impossible to make holes of uniform diameter throughout the length of the hole.
Accordingly, a primary object of the present invention is to provide a flow meter device which affords more uniform flow rates for various types of fluids of differing viscosity and density, and upon which temperature changes has little effect.
Another object of the present invention is to provide a flow meter which will permit rapid and accurate indications of flow rates.
Another object of the present invention is to provide an improved flow meter whose functioning parts can be made with close tolerances so as to provide uniform reliability of use in production quantities of the device.
Yet another object of the present invention is to provide an improved flow meter of simplified construction capable of high volume production at low cost.